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Rosenstein appeared well-prepared, unfazed and responded quickly to all the questions posed by the justices. There was no mention of his special status before the court, as he and the justices stuck to the matter at hand.

While it is unusual for the top officials at the Department of Justice (DOJ) to argue cases before the court, it's not unprecedented. Both Rosenstein and Attorney General Jeff Sessions can argue cases, should they express interest. It is the responsibility of the Solicitor General's office to manage and argue the government's position at the Supreme Court.

For example, Attorneys General Robert Kennedy, Janet Reno and Michael Mukasey all argued cases, and several deputy attorneys general have as well, including James Comey when he held the position, according to DOJ.

Rosenstein, a longtime prosecutor, saw this as an opportunity to flex his lawyer muscles, as his current job is largely administrative and managerial, according to sources familiar with his decision. Arguing before the Supreme Court is also considered one of the greatest honors for an attorney.

"My primary vocation is actually lawyering and not supervising," Rosenstein told the Wall Street Journal in an interview ahead of the arguments.

"A number of Attorneys General and occasionally Deputy Attorneys General have chosen to personally argue a case before the Supreme Court. Not every AG or DAG does it, but many do; and, though Sessions hasn't made an appearance yet, Rosenstein apparently decided he was interested," said ABC News Supreme Court Contributor Kate Shaw.

These arguments are typically in pretty narrow cases, often criminal ones, probably because these top officials have limited time to prepare for a Supreme Court argument, according to Shaw.

Rosenstein made an appearance at the Supreme Court last spring, not for a case, but for Justice Gorsuch's investiture – Sessions was abroad, so Rosenstein delivered Gorsuch's commission from the president.

The case – Chavez-Meza v. United States – that the deputy attorney general argued on Monday focused on sentencing guidelines and whether the lower federal court judge should have provided a more detailed explanation for his sentencing revision.

As a prosecutor, the subject of sentencing guidelines is something Rosenstein would be intimately familiar with. In order to prepare, he worked during his personal time - nights and weekends - to balance his other job duties, sources said.

Adaucto Chavez-Meza and co-conspirators were arrested in 2012 after distributing methamphetamine in New Mexico and involvement with drug cartels in Arizona. In February 2013 Chavez-Meza pleaded guilty to conspiracy and drug possession with intent to distribute – with a federal sentencing guideline of 135-168 months. The government recommended a 135-month sentence and the district court agreed.

A year later, the sentencing guidelines for this type of crime were lowered and Chavez-Meza asked for a reduced sentence. The district court granted a reduced sentence, but still longer than he had argued for.

The district court did not provide "additional explanation" for why it only granted a partial reduction - only indicating that it was "within the amended guidelines." The Tenth Circuit Court upheld the sentence and Chavez-Meza then appealed to the Supreme Court.

If there was an explanation, "we would then have something to grasp onto in appeal," said Chavez-Meza's attorney Todd Coberly.

Rosenstein made his Supreme Court debut amid repeated threats – both public and private – from the president to fire Rosenstein, who oversees Robert Mueller's special counsel investigation into Russia's interference in the 2016 presidential election and possible ties to Trump associates.

Trump has similarly threatened to fire Sessions and Mueller.

However, last week, while responding to a reporter’s question about the fate of the three officials, Trump said, "they're still here."

After the arguments, Rosenstein walked down the front steps of the Supreme Court, past a row of reporters and onlookers. He declined to answer questions about the case, the president or whether he still had confidence in Mueller.

Ben Siu, Jordyn Phelps, John Santucci and Katherine Faulders contributed to this story.